The present invention generally relates to methods and apparatuses adapted to perform additive manufacturing (AM) processes, and specifically, AM processes that employ energy beam to selectively fuse a powder material to produce an object. More particularly, the invention relates to methods and systems that use a pulsed, directed energy beam to achieve predetermined densification and microstructural evolution in AM processes.
AM processes generally involve the buildup of one or more materials to make a net or near net shape (NNS) object, in contrast to subtractive manufacturing methods. Though “additive manufacturing” is an industry standard term (ASTM F2792), AM encompasses various manufacturing and prototyping techniques known under a variety of names, including freeform fabrication, 3D printing, rapid prototyping/tooling, etc. AM techniques are capable of fabricating complex components from a wide variety of materials. Generally, a freestanding object can be fabricated from a computer aided design (CAD) model. A particular type of AM process uses an energy beam, for example, an electron beam or electromagnetic radiation such as a laser beam, to sinter or melt a powder material, creating a solid three-dimensional object in which particles of the powder material are bonded together. Different material systems, for example, engineering plastics, thermoplastic elastomers, metals, and ceramics are in use. Laser sintering or melting is a notable AM process for rapid fabrication of functional prototypes and tools. Applications include patterns for investment casting, metal molds for injection molding and die casting, and molds and cores for sand casting. Fabrication of prototype objects to enhance communication and testing of concepts during the design cycle are other common usages of AM processes.
Laser sintering is a common industry term used to refer to producing three-dimensional (3D) objects by using a laser beam to sinter or melt a fine powder. More accurately, sintering entails fusing (agglomerating) particles of a powder at a temperature below the melting point of the powder material, whereas melting entails fully melting particles of a powder to form a solid homogeneous mass. The physical processes associated with laser sintering or laser melting include heat transfer to a powder material and then either sintering or melting the powder material. Although the laser sintering and melting processes can be applied to a broad range of powder materials, the scientific and technical aspects of the production route, for example, sintering or melting rate and the effects of processing parameters on the microstructural evolution during the layer manufacturing process have not been well understood. This method of fabrication is accompanied by multiple modes of heat, mass and momentum transfer, and chemical reactions that make the process very complex.
Laser sintering/melting techniques often entail projecting a laser beam onto a controlled amount of powder (usually a metal) material on a substrate, so as to form a layer of fused particles or molten material thereon. By moving the laser beam relative to the substrate along a predetermined path, often referred to as a scan pattern, the layer can be defined in two dimensions on the substrate, the width of the layer being determined by the diameter of the laser beam where it strikes the powder material. Scan patterns often comprise parallel scan lines, also referred to as scan vectors or hatch lines, and the distance between two adjacent scan lines is often referred to as hatch spacing, which is usually less than the diameter of the laser beam so as to achieve sufficient overlap to ensure complete sintering or melting of the powder material. Repeating the movement of the laser along all or part of a scan pattern enables further layers of material to be deposited and then sintered or melted, thereby fabricating a three-dimensional object.
In the past, laser sintering and melting techniques have been performed using continuous wave (CW) lasers, typically Nd:YAG lasers operating at 1064 nm. This can achieve high material deposition rates particularly suited for repair applications or where a subsequent machining operation is acceptable in order to achieve the finished object. The method does not, however, lend itself to the production of near-net-shape objects to close tolerances and with a high quality surface finish. Another obstacle that these processes face is the presence of microstructural defects (e.g., voids, impurities, or inclusions) in the final product. Such defects can lead to catastrophic failure.
In view of the above, it can be appreciated that there are certain problems, shortcomings or disadvantages associated with laser sintering and melting techniques, and that it would be desirable if improved methods and equipment were available and capable of producing near-net-shape objects to close tolerances and/or to have high quality surface finishes, and/or capable reducing or eliminating cracks, inclusions, and pores between deposit layers in a finished object.